1. Field of the Invention
The present invention relates to a circuit assembly for charging an accumulator battery with pulsating direct current, containing at least one controllable current valve disposed between the current source and the connecting terminals for the battery and a series resistor provided for generating a reference voltage proportional to the charging current, and also a device, capable of being connected likewise with the connecting terminals for the battery and controlled by the battery voltage, for generating control pulses for the current valve.
2. Description of the Prior Art
For storing electrical energy, accumulators are preferably used which, when being charged, convert the supplied electrical energy into storable chemical energy and, when being discharged, convert the chemical energy back into electrical energy. The quantity of the energy which can be stored in an accumulator and the level of the voltage that can be obtained are dependent upon the construction and dimensions of the accumulator cells, for which reason a plurality of cells are connected in parallel and/or in series to form an accumulator battery for the storage of large quantities of energy and/or relatively high voltages. Such accumulator batteries can be used as mobile energy sources, for example for lifting and transporting vehicles or for the starting of aircraft engines, and also as stationary energy source, for example in emergency power supply installations or in safety installations against fire, burglary and theft.
The conversion of the electrical energy into chemical energy during charging and also the reverse operation during discharging are accompanied by a material change in the composition bothin the electrode surface and also in the electrolyte, and can lead, in the case of unskilled operation, to irreversible damage. This is true especially when the charging or discharging current exceeds a maximum value, but also if charging is continued when the storage capacity of the cell has already been reached. The water in the electrolyte then decomposes, the gas bubbles evolved at the electrodes destroying the fine-pore electrode surface and the increasing acid concentration of the electrolyte"hardening" the electrodes.
The reduction in the chemically stored energy takes place even when the accumulator supplies no current to an external electrical circuit. Although this process takes place very slowly, accumulators for emergency supplies and safety installations, from which no electrical energy is taken during fairly long periods, must be periodically recharged to assure their operating capability, or, better still, be continuously supplied with a maintaining charge, which compensates the autonomous reduction in the chemically stored energy.
In order to prevent damage to a battery by continued charging, it is known to couple the charging device to a clock which, before charging commences, is set as a function of the charging state and the capacity of the battery and, after the set period has expired, interrupts the charging current. Because it is difficult or indeed impossible accurately to determine the charging state of a battery, it is also improbable that, when a charging time preset on the basis of the charging state has expired, the capacity of a battery will be fully charged and the theoretical voltage achieved. In the majority of cases the preset charging time has expired before the battery is charged, or the battery is charged before the charging time has expired and an after-charging of limited duration takes place. It will be understood also that a time-controlled charging operation is not suitable for a maintaining charge.
In order to avoid this disadvantage, a charging device is known from CH-PS No. 572,288, which controls the duration of the charging current as a function of the battery voltage and also makes possible a maintaining charge. This charging device contains a power supply set comprising a rectifier for pulsating direct current and a controlled valve in the one line between the rectifier and the connecting terminal for the battery to be charged. The connecting terminals are additionally connected with the supply voltage lines of a voltage-dependent oscillator, the output signal of which controls the aforementioned valve. In this manner, when an accumulator battery to be charged is connected, the oscillator is energized and the valve is periodically opened for the charging current, and the oscillation and thus also the charging current are interrupted as soon as the voltage of the accumulator battery exceeds the adjustable maximum operating voltage for the oscillator.
This device is not suitable for charging highly discharged accumulator batteries, because the voltage-dependent oscillator not only shuts off when the operating voltage exceeds a presettable value, but also is not energized when the operating voltage supplied by the accumulator battery does not reach a minimum value.
From CH-PS No. 600,646, an improved charging device is known, which is of similar construction to the above-described device and additionally possesses a threshold value switch, which controls the valve for the charging current so long as the voltage of the accumulator battery is smaller than the voltage necessary for energizing the oscillator.
In particular, the last-named device fulfils all the requirements when only one accumulator cell is to be charged. When an accumulator battery, in which one of several cells is defective and therefore the set-point voltage is not reached, is connected, then the charging operation is not interrupted when the functionally effective cells reach their set-point voltage, which can lead to damage or destruction also of these functionally effective cells.